1. Field of the Invention
This invention relates to methods for predicting and maintaining wellbore stability during drilling, well servicing and production in subterranean formations, particularly hydrocarbon bearing subterranean formations.
2. Description of Relevant Art
As used herein, the term xe2x80x9c(in)stabilityxe2x80x9d shall be understood to mean xe2x80x9cstability or instability.xe2x80x9d Wellbore (in)stability in shales is a major problem costing the petroleum industry, according to conservative estimates, $700 million annually. Understanding and modeling mechanisms of shale (in)stability is an ongoing industry effort. Drilling a hole into a formation in equilibrium induces stress concentration in the vicinity of the borehole. Interactions will occur if parameters such as chemical potential, electrical potential, thermal potential, ionic concentration, etc., of the drilling fluid and the shale formation fluid are not in equilibrium. Any differences in these parameters will alter the near wellbore pore pressure which in turn will influence the borehole equivalent stress-state and the shale mechanical strength thus affecting wellbore (in)stability. Tests conducted at compressive uniaxial and triaxial stress conditions in the past have indicated the strong influence of these parameters on shale strength.
Wellbore instability predominantly occurs in chemically reactive shales that overlay reservoirs. A major concern of the drilling engineer is either keeping the borehole wall from collapsing (packing off) or fracturing (losing circulation).
Past efforts to develop improved well bore models (xe2x80x9cWBMxe2x80x9d) for shale drilling have been hampered by a limited understanding of the drilling fluid/shale interaction phenomenon. This limited understanding has resulted in drilling fluids being designed with properties inadequately or insufficiently optimized to prevent the onset of borehole (in)stability problems. Historically, wellbore (in)stability problems have been approached on a trial-and-error basis, going through a costly multi-well learning curve before arriving at reasonable solutions for optimized operations and systems.
Recent studies of fluid-shale interactions have produced fresh insights into the underlying causes of borehole (in)stability; and chemical potential related instabilities in shales have been identified. A chemical potential borehole stability model, derived for an arbitrary borehole orientation, has been successfully implemented to address borehole (in)stabilities. This model, based on a well-established elastic modeling approach, is discussed in Tare, U. A., and Mody, F. K.: xe2x80x9cNovel Approach to Borehole Stability Modeling for ERD and Deepwater Drillingxe2x80x9d, paper SPE 52188, 1999 SPE Mid-Continent Operations Symposium, Oklahoma City, USA, Mar. 28-31, 1999, incorporated herein by reference.
Advances in technology are providing wider and greater capabilities for new, more reliable, accurate and rugged downhole sensors and tools. Such apparatuses and sensors are increasingly able to collect and transmit information to the surface as it is happening.
With increasing economic demands on reducing rig downtimes associated with borehole instability problems, there is increasing need for the ability to predict and resolve instabilities in the field using MWD/LWD (measuring while drilling/logging while drilling) data in conjunction with prior geophysical and shale/fluid interaction knowledge. However, previous attempts have focused on earth stresses, using drilling data and borehole (in)stability modeling on a xe2x80x9cpost-mortemxe2x80x9d basis, i.e., after substantial problems have occurred. There is a need for methods that enable instability problems to be avoided.
A method is disclosed for maintaining subterranean formation stability, and particularly wellbore stability during drilling, well completion and servicing operations, and even during production and enhanced recovery operations. The method of the invention affords maintenance of wellbore stability throughout the life of a well penetrating a subterranean formation. Moreover, the method provides for real-time monitoring of fluid-formation interaction such that adjustments can be made before (in)stability problems occur.
In the method, initial formation characteristics or parameters are estimated or obtained and the rock strength of the formation is estimated. Characteristics of the fluids injected into the wellbore or the formation during drilling, or during well completion and servicing operations, or during production or enhanced recovery operations, are also estimated or obtained. This data concerning the formation and the fluids is input into a well-stability model and results are used to revise the estimates of the formation parameters and/or the fluid parameters or to adjust the fluid composition or characteristics. These parameters and the model results are updated with real time formation measurements taken during the particular operation being conducted (i.e., drilling, well servicing, or production). Continued acquisition and processing of the formation data in the model allows the fluids to be adjusted as needed to avoid formation instability problems caused by interaction of the fluids with the formation.